The present invention relates to lasers and Q-switched techniques, and particularly to a technique for repetitively Q-switching a laser diode-pumped laser using a variable speed moving aperture.
Lasers are devices that generate or amplify light. The beams of radiation that lasers emit or amplify have remarkable properties of directionality, spectral purity, and intensity. These properties have already to led to an enormous variety of applications. The essential elements of the laser device are: 1) a laser medium or gain element consisting of an appropriate collection of atoms, molecules, ions, or in some instances, a semi-conducting crystal; 2) a pumping process to excite these atoms, molecules, etc. into higher quantum mechanical energy levels; and, 3) suitable optical feedback elements that allow a beam of radiation to bounce back and forth repeatedly through the laser medium. The laser resonator contains an optical cavity which is defined by reflecting surfaces that are aligned to resonate the laser radiation within the optical cavity. The laser medium is contained within the optical cavity.
The elements of a laser come in a great variety of forms and fashions. One type of laser gain element that has numerous advantages compared to others is the solid-state laser medium consisting of a laser crystal with one or more types of dopant ions in a predetermined concentration. The solid sate laser gain element is generally optically pumped. The optical pump may be an incoherent source, such as a CW lamp including tungsten filament lamps or arc lamps or pulsed lamps such as flash lamps, or may be a monochromatic source. Monochromatic laser pump sources include ion lasers or dye lasers, or semiconductor laser diodes such as an aluminum gallium arsenide (AlGaAs) laser diode operating at approximately 808.5 nm, which can be used to pump a Nd:YAG (neodymium-doped yttrium aluminum garnet) solid state laser material. As an example of this type of laser see the article, "Efficient Laser Diode Pumped Nd Lasers," by Richard Scheps in Applied Optics, vol. 28 pp.89-91 (January, 1989).
In order for the optical pumping process to be effective, the photons incident on the laser medium must have certain properties. In particular, the pump radiation must be of a wavelength which is absorbed by the laser medium to generate either directly or indirectly the required population inversion for the desired laser transition.
Laser diode pumping of Nd:YAG lasers is well recognized. The laser diode output radiation must substantially match the desired absorption wavelength of the Nd:YAG laser medium which, in general, corresponds to a wavelength of 808.5 nm. This matching of the laser diode emission wavelength with the absorption wavelength and bandwidth of the Nd:YAG laser medium at 808.5 nm is required for efficient operation. Efficient operation in this context considers the pumping efficiency, which is the fraction of absorbed pump photons that populate the upper laser level. Efficient operation also considers the overall electrical power consumption by the pump diodes required to produce a given optical laser power from the Nd:YAG laser. The absorption bandwidth of Nd:YAG is approximately 1 nm.
Two types of diode pumping are generally practiced. The first is called transverse pumping. Transverse pumping describes a technique where the pump flux is incident upon the gain element at an angle, usually 90.degree., with respect to the optical propagation axis of the laser radiation within the optical resonator cavity. The second pumping technique is called longitudinal or end-pumping, and occurs when the pump flux is deposited in the laser gain element parallel to and coincident with the propagation axis for laser radiation contained within the optical resonator cavity. In general, longitudinal pumping of Nd:YAG lasers by laser diodes is preferred over transverse pumping for efficient TEM.sub.00 operation owing to the overlap of the resonator mode with the inversion profile produced by the pump beam. The laser resonator mode describes the spatial distribution of optical energy in the laser resonator. TEM.sub.00 operation describes the lowest order transverse electrical laser resonator mode. This mode of operation is desirable over other transverse modes as it generally requires the lowest threshold power and produces the lowest output beam divergence.
Longitudinal pumping has the potential to provide the lowest threshold power and highest slope efficiency operation of an optically pumped laser. This is because the energy deposition of the pump photons can be located directly within the active volume of the laser gain element. The active volume is determined by the geometry of the optical resonator.
Optical resonators generally consist of two or more flat or curved mirrors set up and aligned to produce optical feedback. The gain medium, which gives each type of laser its name, determines the output power or energy and ultimate tuning range of the emitted radiation. But it is the optical resonator that determines the spatial dimensions of the laser.
A wide range of laser resonator types have been developed and used for laser systems. Some types of optical resonators include plane parallel, confocal, concentric, or hemispherical type resonators. The resonator type is determined by the radius of curvature of the reflective mirrors defining the optical resonator cavity, and the location of each of these mirrors. For the simplest laser resonator cavity containing two reflective elements aligned to form a feedback path between them, the radius of curvature of each of these two mirrors and the spacing between the two mirrors determines the type of resonator. For example, if both mirrors are plane flat mirrors, the resonator type is called plane parallel. A hemispherical resonator consists of a flat mirror and a concave curved mirror separated by the radius of curvature of the curved mirror. In practical lasers, the hemispherical configuration is difficult to achieve because of alignment problems. Generally speaking, a nearly hemispherical resonator, which consists of a flat and curved mirror separated by slightly less than the radius of curvature of the curved mirror, is preferred.
The nearly hemispherical laser resonator mode has a focus or mode waist at the flat mirror, and the mode diameter expands from this waist as the radiation propagates towards the curved mirror. Typically the output coupler, which is the laser mirror though which the radiation is emitted by the laser, is the curved mirror, and the flat mirror is highly reflective (HR). Because the laser resonator mode waist occurs at the HR flat, the power density for the circulating optical radiation is highest at the mode waist. Typically, it is advantageous to place the laser gain element at or near this mode waist, as the extraction efficiency is greatest at this location.
In longitudinal pumping, the pump flux is focused onto the laser gain element and a resonator mode waist is typically located within the laser gain element. The pump efficiency increases as the pump power density increases. For these reasons, the most advantageous orientation for longitudinal pumping is to locate the pump beam waist or focus at or near the laser resonator mode waist within the laser gain element. The diameter of the pump waist should be no greater than the laser resonator mode waist. When the two waist dimensions are approximately equal they are said to be "mode-matched".
Q-switching is a technique which allows extremely high peak power operation of a laser. The Q-switch operates as an intracavity shutter, and remains closed during the time which the gain element is optically pumped. By remaining closed, optical feedback is prevented and radiative losses occur only through spontaneous emission. Typically, the laser gain element is pumped for a time comparable to the spontaneous emission lifetime for fluorescence from the upper laser level. Therefore, losses due to spontaneous emission are minimal and the laser gain element acts as a capacitor, storing the pump energy. Once the gain element is fully "charged", the Q-switch is opened. The intracavity flux builds up to a high peak intensity, and a high energy pulse is emitted by the Q-switched laser. Pulse widths on the order of 5 ns to approximately 40 ns are typically achieved with energies exceeding 1 J. Thus, peak powers of approximately 1 GW can readily be achieved with a Q-switched laser.
There are numerous types of Q-switches including electro-optical, acousto-optical, and mechanical Q-switches. In addition, there are two types of Q-switching. Single-shot Q-switching refers to a technique where the pump excitation is pulsed and the Q-switch opens one time for each pump pulse; therefore, the repetition rate for the Q-switch is determined by the maximum opening rate of the Q-switch or the maximum pulse rate of the pump source, whichever is lower. A second type of Q-switching is called repetitive Q-switching. In this case, the laser gain element is pumped continuously and the Q-switch is opened at a high repetition rate. The Q-switch opening rate is typically 10 kHz or higher, and the maximum Q-switch rate is determined by the desired operating parameters. That is, once the Q-switch opening rate is faster than the inverse of the spontaneous emission lifetime of the laser gain element, then the average Q-switched power is approximately equal to the CW power that would be achieved in the absence of Q-switching. For example, for Nd:YAG with an upper-state lifetime of approximately 200 .mu.s, Q-switch opening rates exceeding 5 kHz produce average Q-switch powers comparable to the CW power that would be achieved under identical pump conditions. Therefore, increasing the Q-switched rate beyond 5 kHz would decrease the energy per pulse from the Q-switched laser. If the Q-switch rate is increased too high then the Q-switched pulse width also begins increase, further reducing the peak power available from the repetitively Q-switched laser.
Electro-optic Q-switches are typically used for single shot Q-switching and operate at rates generally below 100 Hz. Acousto-optic Q-switches are typically used for repetitive q-switching at rates of several kilohertz and higher. Laser diode-pumped Q-switched lasers provide a compact, efficient, high-peak power generating source. However, acousto-optic (AO) Q-switches are not the most effective device for diode-pumped repetitively Q-switched lasers for several reasons. The insertion loss of a Q-switch inside a laser cavity is problematic for diode pumping. Since CW pump fluxes are typically lower than those of other laser pumped sources, the insertion loss for an AO Q-switch arises from two sources. These are bulk scattering problems within the acousto-optic crystal itself and reflections of the laser resonator cavity mode by each surface of the AO Q-switched crystal. In addition, the insertion of an AO Q-switch inside a laser cavity requires expansion of the longitudinal cavity dimensions. For some resonator designs, the insertion of the AO Q-switch introduces astigmatism which lowers the overall optical conversion efficiency. From a pragmatic point of view, AO Q-switches and their associated electronics are expensive. This problem is compounded if different Q-switches with different optical coatings are required for different laser emission wavelength ranges of a given laser gain element.
One of the earliest Q-switched lasers used a mechanical rotating aperture. See R. J. Collins and P. Kisliuk, "Control of Population Inversion in Pulsed Optical Masers by Feedback Modulation, " Journal of Applied Physics, vol 33, pp 2009-2011, 1962. However, the laser operated multi-mode, and no detailed consideration was given to the adjustment of the laser operating parameters in order to optimize the performance of the rotating aperture Q-switch. In addition, optical pumping in the Collins and Kisliuk work was transverse pumping using a flashlamp.
A patent by Kafka et al., U.S. Pat. No. 4,847,850 describes the use of a miniaturized Q-switched laser which is diode-pumped. This patent describes the advantages of Q-switching a diode-pumped laser in that the relatively short cavities characteristic of diode-pumped laser resonators provide short round-trip times and consequently short Q-switched pulses as long as the gain is high and the losses are low. The patent describes the use of several AO Q-switch materials including TeO.sub.2 and LiNbO.sub.3. However, these crystals introduce loss in the cavity, reducing the net gain. The net gain is the gain minus the loss.